One old convert from heathenism (4)
I believe the answer is:
idol
'heathenism' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'one old convert' is the wordplay.
'one' becomes 'i' (Roman numeral).
'convert' is an anagram indicator (I've seen 'converting' mean this).
'old' is an anagram of 'dol'.
'i'+'dol'='IDOL'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for idol that I've seen before include "Hero of screen or stage" , "Pagan symbol" , "Admired figure" , "Object of reverence" , "Pin-up" .)